1. Field of the Invention
The present invention relates to an overturn prevention control device for a two-wheel vehicle, and in particular, to an overturn prevention control device for a two-wheel vehicle capable of traveling autonomously without a human driver.
2. Description of the Related Art
There are known autonomous vehicles without a human driver using an electric motor or an internal-combustion engine as a prime motor and being controlled wirelessly or automatically. When traveling straight, such a vehicle can maintain its balance by steering right for a rightward tilt of the body of the vehicle and steering left for a leftward tilt of the vehicle body. When traveling around a curve, the vehicle can set a target value for a tilt angle of the vehicle body to a direction inclined from a vertical direction, and can steer right for a rightward tilt and steer left for a leftward tilt by using the set angle as the reference. In either case, it is necessary to estimate a tilt angle of the vehicle body.
Japanese Registered Utility Model No. 2577593 describes an autonomous vehicle without a human driver, the autonomous vehicle being capable of stably traveling in a manner that is approximated to an actual machine and in various modes from low to high speeds. This autonomous driverless vehicle includes a frame of a vehicle body, a drive wheel disposed at an end of the frame and rotatable via a primary motor, and a fork mounted to another end of the frame and supporting a steerable wheel so as to allow the steerable wheel to be freely driven and also includes an angular velocity sensor that outputs an angular velocity signal for a fall angle of the vehicle body, an arithmetic unit that generates a steering angle control signal, and an actuator that changes an angle of travel of the steered wheel in accordance with the steering angle control signal output from the arithmetic unit. The arithmetic unit includes an angular velocity command value generating unit arranged to generate an angular velocity command value on the basis of an externally provided travel control signal indicating an angle of travel of the steered wheel, a control signal generating unit arranged to generate a steering angle control signal to be supplied to the actuator on the basis of the deviation between an angular velocity signal being a detection signal of the angular velocity sensor and the angular velocity command value being an output from the angular velocity command value generating unit, and a feedback unit arranged to feed the steering angle control signal generated by the control signal generating unit back to the angular velocity command value generating unit. The actuator generates a steering control signal to control the steered wheel in a direction in which a deviation in fall angular velocity of the vehicle body during travel is reduced in accordance with the steering angle control signal from the arithmetic unit.
When the vehicle body falls, the direction of the steered wheel is controlled in a direction in which the fall angle is reduced. In such an autonomous driverless two-wheel vehicle, the steering angle control signal to be supplied to the actuator is generated based on the deviation between the detection signal of the angular velocity sensor and the angular velocity command value generated based on the externally provided travel control signal indicating an angle of travel of the steered wheel. However, obtaining a proper angular velocity command value from an angle of travel of the steered wheel and obtaining a steering angle directly from the deviation between a detected angular velocity value and the angular velocity command value require complicated computations and many parameters. This leads to a complicated control, which makes it difficult to perform stable autonomous travel.
One example of a relatively simple control method for preventing a two-wheel vehicle from overturning is illustrated in FIG. 7. In this method, an angular velocity ω1 in a lateral direction of inclination of a vehicle body is obtained using an angular velocity sensor 20, the angular velocity ω1 is integrated by use of an integrator 21 to obtain an inclination angle θf in the lateral direction of inclination of the vehicle body, the deviation between the obtained inclination angle θf and an inclination angle command value θr is input into arithmetic means 22 having a proportional gain G1 to generate a steering angle command value δr, and the generated command value δr is output to an actuator 23. This method obtains a steering angle from the deviation in inclination angle using the proportional gain G1. Therefore, it is advantageous in that the computation is simple and, because not many parameters are required, the method is executable in a relatively simple manner.
However, an angular velocity sensor typically has a deviation (drift) in the detection signal due to changes in environmental temperature or a lapse of time, and this has adverse effects such as an offset. Together with the offset, external noise entering the angular velocity sensor 20 affects an angular velocity detection signal. In addition, if the vehicle body is already inclined when the vehicle starts traveling, it affects the inclination angle θf as a zero-set error θ0. Such problems may occur in not only the control method illustrated in FIG. 7 but also the control method described in Japanese Registered Utility Model No. 2577593.
FIG. 8 is an actual control block diagram in which error factors (zero-set error θ0 and offset noise Δ) are added to the block diagram illustrated in FIG. 7. As illustrated in FIG. 8, the zero-set error θ0 is applied to the inclination angle θf, and the offset noise Δ is applied to the angular velocity ω1.
FIG. 9 is an equivalent block diagram into which the block diagram illustrated in FIG. 8 is rewritten. As illustrated in FIG. 9, the zero-set error θ0 is directly applied to the inclination angle command value θr, and the integral of the offset noise Δ is also applied to the inclination angle command value θr. As a result, the zero-set error urges the vehicle body to incline even when the inclination angle command value θr is zero, so the path taken by the two-wheel vehicle is a curve. The integral of the offset noise Δ affects the inclination angle command value θr, and the two-wheel vehicle obeys the inclination angle command value θr including the integral of the offset noise Δ, so the actual inclination angle continues to increase. This causes the two-wheel vehicle to overturn in a short amount of time. As described above, although the control method illustrated in FIG. 7 is a simple control method, a problem in which it is difficult to perform stable autonomous travel due to the zero-set error θ0 and offset noise Δ exists in actual control.